1. Field of the Invention
This invention is directed to flexible printed circuit boards, and particularly, to a device and method for a flexible printed circuit board incorporating stripline or microstrip transmission lines that pass through a small radius bend.
2. Related Art
Flexible printed circuit boards or “flex” circuits are used in a wide variety of applications, where an electrical circuit must bend around corners or be flexed during operation. Typically, flex circuits are thin, light weight, flexible, and exhibit high routability. Generally, a flex circuit may be used as an interconnecting medium in a phased array architecture. In some cases, particularly when microwave signals are present, design considerations mandate that the flex circuit is a stripline construction of certain minimum thickness; which typically consists of a central trace sandwiched between two ground planes, which are spaced a certain distance apart. Two interposing low-loss dielectric material layers are used as insulators. Alternately, the flex circuit may feature a microstrip construction; which typically includes a trace and a single ground plane, spaced a specific distance apart, with a low-loss dielectric material as an insulating interposer.
Generally, there is a minimum bend radius to which flex circuits may be formed without damaging the flex circuit. The minimum bend radius is a function of several aspects of the flex circuit geometry and the materials used, but the distance between the outermost metal layers of the flex circuit is a key parameter limiting the minimum bend radius.
Many flex circuits have only one metal layer, or the distance between the outermost metal layers is minimized, so that the minimum allowable bend radius may also be minimized. Unfortunately, in some cases the distance between the outermost metal layers cannot be decreased below a particular value due to electrical design considerations or manufacturing limitations. This is often the case with flex circuits that incorporate a stripline or microstrip construction.
When a flex circuit having two or more metal layers is formed to a bend radius that is less than allowable minimum, the external copper layers of the circuit tend to crack or buckle. Internal delamination has also been observed. In some cases concerning a flex circuit with a stripline construction, one or more central traces have broken, resulting in open circuits. This results in low manufacturing yields, and raises serious long-term reliability concerns. Typically, the copper ground plane on the convex side of the flex circuit cracks while the copper ground plane on the concave side buckles. When no cracking occurs, it is often because internal delamination has provided strain relief, sufficient to prevent cracking, but such delamination leads to additional reliability problems.
What is needed is a structure and method that allow bending of the flex circuit around a small radius while preserving both the mechanical and electrical integrity of the design.